Riyad as-Salihin, Book 18, Hadith 1559 (In-Book) ⓘ
Ibn Mas'ud (May Allah be pleased with him) reported: The Messenger of Allah said, "Reviling a Muslim is Fusuq (disobedience of Allah) and killing him is (tantamount to) disbelief."[Al-Bukhari and Muslim].
وعن ابن مسعود رضي الله عنه قال: قال رسول الله صلى الله عليه وسلم : "سباب المسلم فسوق، وقتاله كفر" ((متفق عليه)).